1. Field of Industrial Application
This invention relates to an electron tube with an electron multiplier for multiplying the flow of incident electrons by secondary electron emission.
2. Related Background Art
Electron tubes for multiplying the flow of incident electrons by secondary electron emission conventionally known are electron multipliers, photomultipliers and image intensifiers, etc. The electron multipliers disposed in these electron tubes usually comprise a plurality of stages of dynodes with secondary electron emission.
A sectional view of the dynodes constituting one of these electron multipliers is shown in FIG. 1. In FIG. 1 the n-th stage and the (n+1)-th stage laid on the n-th stage are extracted out of a plurality of stages of dynodes which are laid one on another and which are electrically insulated from one another.
The dynode 80 of each stage includes a plate 82 having a plurality of through-holes 81 formed therein. The plate 82 of each stage is turned with respect to that of a next stage so that the through-holes 81 of the former stage are directed opposite to those of the latter. The plates 82 of the respective stages are supplied with predetermined voltages by power sources 83 associated with the respective stages so that the dynodes 80 of the respective stages have gradually increased potentials. In the case of FIG. 1, V.sub.1 =100 V, and V.sub.2 =200 V. The surface of each plate 82 including the inner surfaces of the through-holes 81 are electrically conducting, and the entire surface of each plate 82 is charged with the same potential by a voltage applied thereto.
When electrons are incident on the n-th one of the thusarranged stages of dynodes, electrons incident on the through-holes 81 impinge on the slant surfaces 84 of the through-holes 81, and secondary electrons are emitted from secondary electron emitting layers formed on the slant surfaces 84. The emitted secondary electrons are guided by a control electric field formed by a potential difference between the n-th and the (n+1)-th stages to be incident on the (n+1)-th dynode and multiplied there again in the same way.
Distributions of the potential between the n-th and the (n+1)-th stages are shown by the dot-lines in FIG. 1. For example, equipotential lines of 120 V, 150 V and 18O V are shown and indicated respectively by A, B and C. The equipotential line B is located intermediate between the n-th and the (n+1)-th stages, and the equipotential line A and the equipotential line C are curved respectively in the through-holes of the n-th stage and in those of the (n+1)-th stage.
As described above, the secondary electrons emitted from the n-th dynode 80 are guided by a control electric field formed by a potential difference between the n-th and the (n+1)-th stages to be incident on the (n+1)-th stage dynode 80. But in such conventional dynodes, the curve-in of the equipotential line into the through-holes 81 of the n-th stage, which functions as a control electric field, is insufficient. It is a disadvantage that the control electric fields in the through-holes are weak. As a result, emitted secondary electrons often adversely return to the n-th stage, which is one cause for lowering the electron collecting efficiency.